1. Field of the Invention
The present invention relates to a developing device for developing a latent image formed on an image carrier with a developing liquid consisting or toner or similar developing substance and a carrier liquid, and a copier, facsimile apparatus, printer or similar image forming apparatus including the same. More particularly, the present invention relates to a developing device including a reservoir storing the developing liquid and an agitator rotatable in the reservoir for agitating the developing liquid, and an image forming apparatus including the same.
2. Description of the Background Art
A developing device of the type using a developing liquid, which consists of toner or similar developing substance and a carrier liquid, is conventional. This type of developing device includes a cylindrical reservoir storing the developing liquid and a developing roller or similar developer carrier. The developer carrier conveys the developing liquid deposited thereon to a developing position where a photoconductive drum or similar image carrier is located. The developing liquid is transferred from the developer carrier to a latent image formed on the image carrier at the developing position, thereby developing the latent image. This kind of configuration is taught in, e.g., Japanese Patent Laid-Open Publication No. 11-352783.
The developing liquid customarily stored in the reservoir is thixotropic, i.e., it has relatively high viscosity and contains the developing substance by a content of 5% to 40%. The viscosity of the thixotropic liquid decreases little by little when subjected to a shearing force, but increases little by little when freed from the shearing force. An agitator implemented as a paddle is disposed in the reservoir and rotatable for agitating the developing liquid. The paddle in rotation causes the developing liquid to swirl and lower its viscosity little by little. At the same time, the toner or similar developing substance (simply toner hereinafter) is evenly dispersed in the entire developing liquid, providing the liquid with a uniform toner content.
A float type liquid level sensor is sometimes disposed in the reservoir for sensing a liquid level in the reservoir, so that preselected control is executed in accordance with the sensed liquid level. Typical of this type of liquid level sensor includes a magnet or similar magnetic force generating means, Hall elements, magnetic switches or similar magnetic force sensing device, and a float movable up and down in accordance with the liquid level. One of the magnetic force generating member and magnetic force sensing device is affixed to the float and movable up and down together with the float while the other of them is affixed to the reservoir. A distance between the magnetic force generating member and the magnetic force sensing device varies due to the up-and-down movement of the float. This allows the magnetic force sensing device to sense a liquid level in the reservoir.
The problem with the conventional float type liquid level sensor is that the float is sometimes positioned at a level different from the actual liquid level in the reservoir. For example, while the top of the float is usually positioned above the liquid level, the developing liquid deposits on the float due to the waves and eddies of the liquid. When only the liquid evaporates or drops along the side of the float, the toner is left on the top of the float. The toner sequentially accumulates on the top of the float due to repeated deposition, evaporation and drop, causing the float to sink due to the weight of the toner. Consequently, the float is positioned at a level different from the actual liquid level. Particularly, the toner easily accumulates on the top of the float when the developing liquid is thixotropic.
Further, the developing liquid in the reservoir usually flows due to agitation during operation, making the liquid level inconstant. More specifically, the liquid surface noticeably waves or sequentially rises outward from the center of agitation. When the actual level at the position of the float differs from the actual liquid level (still state), the float is positioned at a level different from the actual liquid level. As a result, the magnetic generating member or the magnetic force sensing device connected to the float is, of course, located at an unexpected level, producing a difference between the actual liquid level and the sensed liquid level.
Moreover, an eddy appears at the center of the developing liquid in the cylindrical reservoir. The eddy lowers the liquid level at the center while raising it at the periphery. In addition, friction acting between the developing liquid and the wall of the reservoir generates a force that obstructs the swirl of the liquid and thereby causes the liquid surface to wave. This also results in the difference between the actual liquid level and the sensed liquid level.
A toner content sensor, for example, is often disposed in the reservoir for sensing the toner content of the developing liquid. In this case, the paddle or agitator is located at a position offset from the center of the reservoir, so that the paddle does not contact the toner content sensor. The paddle should therefore be short enough for layout reasons and cannot sufficiently agitate the developing liquid in the reservoir. Furthermore, the paddle cannot sufficiently agitate the developing liquid in the up-and-down direction although it can agitate it in the horizontal direction.
In the developing device, the developing liquid is left on the developer carrier after development and on the image carrier after image transfer. Such residual liquids should preferably be collected by respective cleaners and returned to the reservoir to be reused. The collected liquids, however, often have toner contents different from the original content before development and therefore cause the toner content of the developing liquid stored in the reservoir to vary.
To solve the above problem, an arrangement may be made such that a carrier liquid, toner or similar developing substance, developing liquid or similar control agent is replenished to the reservoir in accordance with a toner content sensed by a toner content sensor, as proposed in the past. With this configuration, even when the collected liquids are returned to the reservoir and vary the toner content of the developing liquid in the reservoir, the control agent allows the liquid to restore its original toner content.
The thixotropic developing liquid is capable of forming an image in a smaller amount than a developing liquid whose standard toner content is lower than 5%. However, it is difficult to stabilize the toner content of the thixotropic developing liquid stored in the reservoir for the following reason.
It is a common practice with the developing device to add a control agent whose toner content is higher than the standard toner content to the developing liquid, which is diluted by the collected liquids. So long as a traditional developing liquid whose standard toner content is 1% or below is used, a thixotropic developing liquid having a toner content of about 10% can rapidly restore the standard toner content when added as a control agent. This is because a developing liquid whose toner content is far higher than the standard toner content can be added as a control agent and can therefore restore the standard toner content when added in a small amount. On the other hand, even when the developing liquid is thixotropic and has a toner content of 5% to 40%, a control agent whose toner content is higher than the standard content must be added when the developing liquid is diluted. However, the control agent cannot exhibit the properties of a liquid unless the toner content thereof is limited. It follows that the control agent must be replenished in a far greater amount than the traditional low toner content, low viscosity developing liquid. However, the amount of control agent that can be replenished to the reservoir is limited. Even if the control agent is replenished to the upper limit, then it often fails to effectively increase the toner content of the developing liquid in the reservoir. This makes it difficult to stabilize the toner content of the developer in the reservoir.